1. Field of the Invention
The present invention relates to an optical phase difference control system and method, and an optical signal transmitter that control a phase difference of an optical time-division multiplexed (OTDM) intersymbol carrier wave.
2. Description of the Background Art
For example, in a high-speed optical transmission system with a transmission rate of 40 Gbit/s or more, if longer-distance transmission is to be performed, the input intensity of an optical signal must be increased. However, an increase in the input intensity will cause waveform degradation due to a nonlinear effect in optical fibers, resulting in transmission quality degradation.
To prevent the waveform degradation, CS-RZ (Carrier Suppressed Return-to-Zero) modulation is utilized, which has the advantage of being able to suppress a nonlinear effect in optical fibers to enhance dispersion proof stress and other advantages. CS-RZ modulation is the technique of inverting an optical carrier phase between adjacent pulses, i.e., adjacent bits, in an optical signal pulse train. The CS-RZ modulation is able to suppress high-level carrier frequency components contained in NRZ (Non-Return-to-Zero) signals or RZ (Return-to-Zero) signals and generate pulse signals of which the occupied bandwidth is suppressed to about one-half as broad as the NRZ or RZ signals. In this manner, even when long-distance transmission is performed, it is less susceptible to waveform degradation due to optical fiber wavelength dispersion or optical nonlinearity.
CS-RZ modulation has many combinations of optical carrier phases between adjacent pulses. Among them, the combinations, (0, π, 0, π), (0, π/2, 0, π/2), (0, 0, π, π), and (0, 0, 0, π) are less liable to cause degradation due to a nonlinear optical effect during fiber transmission. For instance, the combination (0, π, 0, π) means the multiplexing of four waves. If the four waves are represented by a first, a second, a third and a fourth wave, respectively, the numerical values in the combination (0, π, 0, π) represent the phase differences of the carrier light-wave of the first, second, third, and fourth waves with respect to the phase of the first wave. The combination (0, π, 0, π) means that the phase differences of the carrier light-wave between the first and second waves, between the second and third waves, and between the third and fourth waves are all π (pi) in radian.
If any of the above-described four phase relationships cannot be preserved and therefore a shift in phase occurs, degradation due to a nonlinear optical effect will be caused. If phase relationships are to be preserved, an optical carrier phase difference between adjacent pulses has to be controlled. A conventional optical phase control system and optical signal transmitter are disclosed in Japanese patent laid-open publication Nos. 2004-23537, 2005-6176, and 2005-6174 by way of example, hereinafter referred to as Documents D1, D2 and D3, respectively.
In the inventions disclosed in Documents D1 and D2, the combination (0, π, 0, π) is adopted as a combination of optical carrier phases. To perform the detection and control of an optical phase difference between bits, part of a signal to be transmitted is taken out and input to an interferometer. In those two prior art documents, to detect and control such a carrier phase difference between bits, part of a modulated optical signal, i.e. CS-RZ modulated optical signal, is taken out and split into two optical signals. Subsequently, with a delay unit, between the two split modulated optical signals, theirs carrier waves are given a phase difference equivalent to the amount of, for example a one-bit delay. Next, by multiplexing the two waves, the carrier waves of adjacent bits are caused to interfere with each other. The interference light-wave is monitored; the time average value of the optical power thereof is converted into an electrical signal (monitored voltage); and based on the converted value, the carrier phase difference is detected and controlled.
In the two documents, i.e. D1 and D2, the carrier phase difference between adjacent bits in CS-RZ modulation is in a state of π(pi), i.e., the state in which the phases of the carrier waves are inverted with each other. When the carrier waves are caused to interfere with each other after being given a delay of one bit in an interferometer, if the phases of the carrier waves of the modulated optical signals have no phase shift, the phases of the carrier waves are inverted with each other. Therefore, because of light-wave interference, they cancel out each other and become extinct. On the other hand, when the carrier waves are caused to interfere with each other after being given a delay of one bit in an interferometer, if the phases of the carrier waves of adjacent modulated optical signals are shifted from a predetermined value by π, the phases of the carrier waves are the same. Therefore, because of light-wave interference, they strengthen each other. This means that when the carrier phase difference between bits of 1-bit delayed interference light-wave is π, a monitored voltage based on the optical power of interference light-wave is a minimum, and when the carrier phase difference is zero, the monitored voltage takes its maximum.
Therefore, as the carrier phase difference between bits of a modulated optical signal is shifted from a predetermined value, π, the monitored voltage of the multiplexed optical output signal, i.e., interference light-wave becomes higher. When the carrier phase difference between bits is shifted by π and becomes 0, the multiplexed output light-wave, i.e. interference light-wave, is most strengthened, and consequently, the monitored voltage takes its maximum.
Hence, if the carrier phase difference between bits changes from π to 0, the time average value of multiplexed output light-wave changes from its minimum to maximum. If the time average value is monitored and fed back to a difference in optical path length, control of the carrier phase difference between bits becomes possible in the two documents. In addition to control of the carrier phase difference between adjacent bits, both documents have proposed that the carrier phase difference between bits that are two or more bits away from each other is controlled in the same manner.
In the two documents, to generate a signal having a phase relationship of (0, π, 0, π), two light waves A, which have a phase relationship of (0, 0) in which there is no phase difference, are generated and two light waves B, which have a phase relationship of (π, π) in which the phase difference is π with respect to the light waves A, are generated. Next, the light waves A and light waves B are temporally shifted and combined into a single signal with a phase difference of (0, π, 0, π).
Well, the remaining document, D3, discloses that when a signal with a phase relationship of (0, 0, 0, π) is received, a clock signal synchronized with a transmission signal is generated by a interference light-wave between the received signal and this signal after being delayed a period of one bit, as previously described. In the method of reception disclosed in Document D3, attention is directed to the fact that in the case of a signal with a phase relationship of (0, 0, 0), interference light-wave, which is obtained between that signal and a signal in which the phase relationship of (0, 0, 0) is respectively shifted one bit by bit, contains a portion in which phase differences of (0, 0) overlap. In other words, a frequency component equal to half of that of the received signal is obtained from that overlapping portion. Note that Document D3 does not disclose phase difference control of transmission signals.
The transmission signal described in Document D3 can be generated by the transmitter disclosed in Document D1 or D2. However, the use of the transmitter makes it difficult to perform phase difference control. This problem will be described hereinafter.
In the case where the conventional system disclosed in Document D1 or D2 is used when generating an optical phase condition of (0, 0, 0, π) disclosed in Document D3, four separate signals with a transmission rate of x bit/s to be multiplexed are employed in order to generate four separate signals that have a transmission rate of 2x bit/s, which is twice as high as the former. In Document D2, four separate signals are generated so that the phases of the carrier light-wave respectively have (0, π) and (0, 0). The four separate signals are multiplexed into a single signal with a transmission rate of (4x bit/s) equal to four times as high as the transmission rate (x bit/s) of each separate signal.
Since optical phases before being multiplexed are preserved at (0, π) and (0, 0), if a phase difference of φ is present between two optical signals with a phase relationship of (0, π) and two optical signals with a phase relationship of (0, 0), the 4x-bit/s multiplexed optical signal has a phase relationship of (0, φ, π, φ), where the φ denotes the phase difference between two 2x-bit/s signals after they are multiplexed to a 4x-bit/s signal. As shown in FIG. 2, the 4x-bit/s signal interferes in phase differences of (φ), (π−φ), (φ−π), and (−φ) at 1-bit intervals, within the interferometer disclosed in Document D2. FIG. 2, line (a), shows the relative phase of each bit of a transmission signal, that is, carrier light-wave of each channel, with the fourth channel as reference. FIG. 2, line (b), shows the relative phases of a signal obtained by delaying each bit of the transmission signal of FIG. 2, line (a), by one bit. FIG. 2, line (c), shows the relative phases of a signal obtained by causing the signals shown in FIG. 2, lines (a) and (b), respectively to interfere with each other. The relative phase shown in FIG. 2, line (c), is a difference in phase between the signals shown in lines (a) and (b).
FIG. 3 shows intensity changes of interference light-wave with respect to an increase or decrease in φ. The horizontal axis of FIG. 3 represents φ (radian), while the vertical axis represents intensity. A curve 100 shows intensity changes of interference light-wave whose phase difference is φ or −φ. A curve 102 shows intensity changes of interference light-wave whose phase difference is (π−φ) or (φ−π). As shown by the curve 100, the intensity of interference light-wave whose phase difference is φ or −φ changes in the same manner with an increase or decrease in φ. As shown by the curve 102, the intensity of interference light-wave whose phase difference is (π−φ) or (φ−π) changes in the same manner with an increase or decrease in φ. The curve 100 and curve 102 change to cancel out each other with respect to an increase or decrease in φ. Considering that in a normal digital transmission signal the ratio between 1s and 0s is ½, the resultant average output intensity does not change even if φ fluctuates. That is, the conventional technique disclosed in Document D1 or D2 cannot achieve the stability of the transmission signal disclosed in Document D3. The reason why Document D1 or D2 can control a phase difference is that since Document D1 or D2 uses a signal with a phase relationship of (0, π, 0, π), the phase difference of interference light-wave between a 1-bit delayed signal and a signal before being delayed is always (π−φ) or (φ−π), not φ and −φ. That means that, since interference light-wave equivalent to the curve 100 does not occur, there is no possibility that the curve 100 and curve 102 will cancel out each other.